The present invention relates to a hydrogen storage tank, and more specifically to a hydrogen storage tank that accommodates a hydrogen storage alloy and a heat exchanger.
In recent years, awareness of suppressing global warming has been raised. Particularly, development of fuel cell cars and hydrogen engine cars has been promoted to reduce carbon dioxide emitted from vehicles. In a fuel cell car, hydrogen and oxygen are electrochemically reacted with each other to generate electricity. The generated electricity is supplied to a motor to generate driving force. As a method for supplying hydrogen, there is known a technique in which a hydrogen storage alloy is accommodated in a tank, which alloy stores and releases hydrogen to be utilized. A hydrogen storage alloy generates heat when storing hydrogen and absorbs heat when releasing hydrogen. It is therefore common to provide a heat exchanger in a tank accommodating a hydrogen storage alloy to promote storage and release of hydrogen. For example, a heat exchanger having heating medium tubes through which a heating medium flows and comb-like fins provided on the circumference of the tubes is used. Instead of heating medium tubes, some heat exchangers have corrugated plates and flat plates that are combined to form passage for a heating medium (for example, Japanese Laid-Open Patent Publication No. 2000-111193).
Incidentally, a hydrogen storage alloy is used in the powder form. Also, a hydrogen storage alloy has a property to be inflated when storing oxygen. Hydrogen storage alloy powder may be settled and compacted in the tank. When hydrogen is stored by such compacted hydrogen storage alloy powder, the alloy powder is expanded, which locally generates an excessive stress. This can adversely affects the tank main body and the heat exchanger in the tank. As measures against such drawbacks, sheet-like members are provided between fin tubes forming a heat exchanger, for example, in Japanese Laid-Open Patent Publication No. 11-43301, to prevent hydrogen storage alloy powder from sedimenting. Also, Japanese Laid-Open Patent Publication No. 2001-10801 discloses a structure in which heating medium tubes having an oblong cross-section prevent hydrogen storage alloy from sedimenting.
Also, Japanese Laid-Open Patent Publication No. 2003-120898 discloses a metal hydride containing device in which containers each containing a predetermined amount of metal hydride are stacked in a main body of the device. The device is configured to have spaces to allow the metal hydride to expand when storing hydrogen.
However, although the technique disclosed in the publication No. 11-43301, which has sheet-like members between fin tubes, prevents hydrogen storage alloy powder from sedimenting, a space for accommodating the sheet-like members must be saved in the tank main body. Accordingly, the space for accommodating hydrogen storage alloy powder and the heat exchanger is reduced. Also, since the fin tubes and heat exchanger fins are arranged along a cross-sectional direction perpendicular to the longitudinal direction of the tank main body, heat exchanger fins not contact the hydrogen storage alloy powder in many sections. In this manner, although the technique disclosed in the publication No. 11-43301 is capable of preventing hydrogen storage alloy powder from sedimenting in a tank, the heat exchanging performance between the hydrogen storage alloy powder and the heat exchanger is degraded.
On the other hand, according to the technique disclosed in the publication No. 2001-10801, the drawbacks of the publication No. 11-43301 may be eliminated by preventing a hydrogen storage alloy from sedimenting by using the heating medium tubes. However, since the tubes have curved sections, the tubes have the limits in terms of prevention of hydrogen storage alloy sediment. The method of the publication No. 2001-10801 is therefore hardly reliable.
In Japanese Laid-Open Patent Publications No. 2000-111193, No. 11-43301, and No. 2001-10801, increasing the amount of hydrogen storage alloy powder filling spaces between fins causes the powder to be pulverized when storage and release of hydrogen are repeated. Accordingly, the bulk density of the powder is reduced. In such a case, the heat exchanger can be deformed or damaged if excessive pressure in a direction perpendicular to the fins is applied. None of the publications discloses measures against such a drawback.
On the other hand, a storage container disclosed in Japanese Laid-Open Patent Publication No. 2003-120898 is designed in consideration of expansion of metal hydride when storing hydrogen into the metal hydride. However, the publication has no mention of reduction of the bulk density due to pulverization.